>To me, that means that the pages are of
>different sizes - Brand X must put fewer characters per page.

The edition reffered to by the CJK-IRG was a recent one based on
the Tong-Wen shu-ju edition, reproduced by the Zhong-Hua shu-ju.
The original Kan-Xi editions contain approx. 3500 fol (7000 pgs).

Your `Brand X' may be a recent reproduction of the revised version
issued in DaoGuang period.

NB. 1. unfortunately, there seems to be no published reproduction of
the `original' editions of Kan-Xi. (or is there ?)
2. the `original' edition is not one, but are set of versions
minutely different from each other, among which errata are
silently corrected, characters and notes are supplemented/
deleted, and (of course) glyphs are (silently) replaced.
Just look at JIS X0208:1997 52-46 (its rationale), cf. UCS 583d.